Apparatuses of the type in question are used particularly in order to convert analog signals into digital signals, for example in order to be able to evaluate and further process signals delivered by a sensor by means of electronic data processing in a microcontroller or another control unit. The distortion signal generator is used in this case particularly to increase the resolution of the analog-to-digital converter. The background to the use of such a distortion signal generator is the fact that although many analog-to-digital converters have a sampling frequency that is much faster than required for a particular purpose, their resolution, that is to say the minimum distinguishable signal level, means that at the same time they allow only excessively coarse capture of the signal. For analog-to-digital converters, the resolution is usually indicated in bits.
By way of example, analog-to-digital converters can have a sampling rate of 100 kHz to 200 kHz, whereas the bandwidth of measured variables from a connected sensor, particularly in the automotive sector, may be below 1 kHz, for example. In this case, the resolution is frequently in the order of magnitude of approximately 10 bits, whereas resolutions of 13 bits or 14 bits, for example, would be desirable in numerous applications.
The distortion signal generator is therefore used to perform a conversion of the temporal resolution into a higher sampling accuracy. This method is also referred to as dithering. This involves the signal to be digitized having noise with a low RMS value added to it, typically in the order of magnitude of a few least significant bits (LSB). A resultant signal that is then digitized by the analog-to-digital converter fluctuates by the value corresponding to the signal to be digitized having the instantaneous value of the distortion signal. If the distortion signal has the mean value zero, for example, then the mean value of many output values from the analog-to-digital converter can correspond to the correct output value of the constant signal to be digitized. This mean value has a higher resolution, however. The reason is that mean value formation gives rise to digits having a lower value than 1 LSB. In principle, the use of both random noise and pseudo noise as a distortion signal is available for this purpose. In the case of random noise, there is no absolute certainty that the mean value for a group of, by way of example, fifty successive values of the noise is actually also zero. Random noise of this kind can be generated by a random number generator, for example, which is based on the observation of a natural phenomenon such as radioactive decay, for example. The use of random noise typically involves the use of a mathematical function that generates pseudo random numbers on the basis of an output value. These pseudo random numbers have defined properties; in particular, it is possible to compute how many pseudo random numbers need to be grouped in order to achieve a mean value of zero. Even if the mean value of such a group is not zero, the pseudo noise can still be used, since the mean value is known and can simply be compensated for computationally.
When using random noise, reference is particularly also made to “random dithering”, whereas “deterministic dithering” is also referred to when using pseudo random numbers. It has been found that when pseudo random numbers are used, it is also possible to use a simply defined signal such as a triangular waveform signal or sawtooth signal, for example. If a mean value of zero is intended to be achieved in this case, this can typically be accomplished by virtue of the period from which the values used come, which are averaged to form a value, comprising an integer number of periods of the relevant distortion signal. In principle, however, it even suffices if the mean value is constant and known, since there is the possibility of computational compensation.
In the case of apparatuses of the type in question, the implementation of deterministic dithering typically involves the use of a distortion signal generator, an adder, an analog-to-digital converter and a microcontroller. In this case, the adder typically caters for superimposition of the distortion signal over the analog signal to be digitized, whereupon said signal is digitized by the analog-to-digital converter and forwarded in digitized form to the microcontroller. The microcontroller subsequently forms the mean value over a number of measured values, resorting to information from the distortion signal generator. Adders are typically designed using active components such as operational amplifiers. It has been found that although embodiments of this kind achieve a high resolution and are suitable for numerous purposes, they are also, on the other hand, complex and expensive to produce.